The present invention relates generally to centrifugal casting apparatus, and is particularly concerned with the type of apparatus wherein molds of rubber or the like are positioned in centrifugal casting machines so as to cast small metallic parts, such as jewelry and the like.
Centrifugal casting utilizing rubber molds is extremely old and well known, particularly in the jewelry field. The molds, which are constructed of a rubber-like material, such as natural rubber, silicone, SBR, or the like, traditionally comprise two circular mold halves having mold cavities and radially extending gates formed on their abutting surfaces so that when the mold halves are clamped together in the casting machine, said abutting surfaces cooperate with each other to define the desired mold cavities with gates extending radially inwardly from said cavities. In the traditional mold arrangement, the upper mold half has a circular opening at its central portion, while the lower mold half comprises a complete disc, i.e., it has no such central opening. The mold halves are positioned in the casting machine with the working surfaces 1 thereof in abutting relationship with the mold halves positioned between a movable piston plate and a pressure plate, whereby movement of the piston plate by pneumatic power means or the like clamps the two mold halves against said pressure plate whereupon said mold halves are tightly clamped against each other. Locating means are provided on the inner working surfaces of the mold halves to insure proper orientation of one half with respect to the other. At this point, and with the machine spinning or rotating by conventional drive means, molten metal is poured, usually manually, into the space defined by the central opening of the upper mold half, whereupon centrifugal force causes the molten metal to flow radially outwardly through the radially extending gates to the mold cavities until said cavities are completely filled with the molten metal. At this point the metal is allowed to solidify, the molds are separated, and the castings are removed therefrom, after which the gate castings are broken away from the cast articles, and the latter are then finished by conventional techniques to remove surface irregularities and the like. This process has long been used in the jewelry industry to produce relatively inexpensive jewelry castings of so-called white metal, although other alloys can be used.
Although generally satisfactory, the above described centrifugal casting process has a number of disadvantages. First of all, to insure proper casting, substantial clamping pressure must be applied to the mold halves, which pressure, because of the rubber-like material of which the molds are made, results in some degree of distortion of the mold cavities. This distortion is somewhat amplified by the relatively high rotational speed that is required to insure sufficient centrifugal force to fill the mold cavities. Obviously this distortion of the mold cavities results in comparable distortion of the cast parts which, of course, is highly undesirable, and while such distortion may be something that one can live with when dealing with relatively inexpensive cast jewelry, it effectively prevents mechanical parts having any kind of precision requirements, such as nuts and bolts, for example, from being satisfactorily made by the centrifugal casting process.
In addition, a common problem with rubber molds of this type is so-called "flashing",i.e., erosion of the surfaces of the molds adjacent the mold cavities which permits small amounts of molten metal to be forced outwardly from the mold cavities between the mold halves. It is conventional to minimize this "flashing" phenomenon by placing shims at the outer surfaces of the mold halves to increase the clamping pressure at those areas of the mold where "flashing" is taking place.
Another problem is that the mold cavities must be back-vented, i.e., vents must be provided to receive the air that is forced out of the mold cavities when the molten metal flows therein.
Aside from distortion, "flashing", and back-venting, conventional rubber molds for centrifugal casting require a substantial degree of skill from the person operating the casting machine. More specifically, proper clamping pressure must be applied to the mold halves, appropriate shims must be inserted, and the rotational speed of the casting machine is quite critical, as is the amount of molten metal that is manually poured into the mold. Thus, the effectiveness of the conventional centrifugal casting process is to a large extent dependent on the skill of the particular operator involved.
Finally, since it has heretofore been conventional to place only one mold in the casting machine for each molding operation, the time required to produce the desired number of cast parts is necessarily magnified.